Our studies of Tregs have revealed that TNF by acting on the TNFR2 receptor, which is most highly expressed by Tregs, unexpectedly results in the proliferative expansion and functional activation of Tregs both in mice and in man. In view of the well-known proinflammatory effects of TNF, our data showing that TNF in a more delayed manner can also down-regulate immune responses is rather surprising. Furthermore, TNF together with IL-2 up-regulates the cell surface expression of TNFR2 and also of 4-1BB and OX-40 receptors. Thus, TNF amplifies its stimulatory effect on Tregs by inducing 3 TNFRSF members. TNF interactions with TNFR2 also stabilizes the CD4+FOXP3+ T regulatory cell phenotype at inflammatory sites. One clarification of these unexpected effects of TNF is based on our data showing that TNF by activating Teffector cells also induce them to express more TNFR2 and to become more resistant to the suppressive effects of Tregs. Thus activated inflammatory sites can prevail over the suppressive effects of Tregs. However, as inflammation subsides in healing wounds or in non-inflamed tumors, Tregs prevail. About 50% tumor infiltrating T cells (TIL's) develop into Tregs and express TNFR2. They are activated by tumor-derived TNF to be even more immunosuppressive than Tregs in peripheral lymphoid tissues. Suppression of Tregs should enable more effective host antitumor responses to be enabled to suppress tumor growth. We hypothesize that TNFR2 functions as a checkpoint inhibitor of Tregs. This hypothesis was tested by us by treating mice bearing 4T1 mammary tumors with neutralizing anti-TGFbeta and cyclophosphamide, which suppresses Tregs, thus promoting cell mediated immunity. This resulted in 80% of the mice becoming tumor free and preferentially resisting a subsequent 4T1 tumor challenge. These treated mice developed considerable infiltration of their tumors by IFNgamma producing rather than Treg TIL cells and also reduced the numbers of MDSC in their spleens and tumors. These finding suggest that this therapy enabled the mice to exhibit considerable antitumor immunity. During the past year we showed that the human MT-2 cell line has the properties of Tregs and can be used as a target of suppressors of Tregs and to bioassay the functional effects of Tregs. In addition, in collaboration with Dr. Yinling Hu, we showed that the mechanism of action of Tregs required signal transduction by IKKalpha.